1. Field of the Invention
The present invention relates to a magneto-optical element having the Faraday effect and an optical magnetic field sensor composed by the use of the element.
2. Description of Prior Art Example
Recently, in particular, in the field of power industry, there has been proposed and commercially applied a magnetic field measuring equipment combining a magneto-optical element having the Faraday effect with an optical fiber, as a method of measuring a magnetic field intensity developing around an electric wire by the use of a light. The method of measuring a magnetic field intensity around a conductor through which a current flows to detect the current is characterized in that, for example, the method has good insulation properties because it uses the light as a medium, and is not subjected to an electromagnetic induction noise, so that the method is considered to be used for transmission/distribution facilities.
FIG. 3 shows a principle diagram of a method of measuring a magnetic field by the use of the Faraday effect. In FIG. 3, a magneto-optical element 14 is arranged in a magnetic field H. An incident light 8a is converted to a linearly polarized light by a polarizer 13 and is caused to pass through the magneto-optical element 14. The plane of polarization thereof is rotated in proportion to the magnetic field intensity H by the Faraday effect. FIG. 3 shows a case where the Faraday rotation exhibits a negative code. The linearly polarized light having been rotated passes through an analyzer 15 whose transmission-polarization direction is made different by 45 degrees from that of the polarizer 13, and then converted in the magnitude of the rotation angle .theta. thereof to a change in the intensity of an emitted light 8b. In order to compose this magneto-optical converting section, there is generally used an optical magnetic field sensor composed as shown in FIG. 2 (see National Technical Report Vol. 38, No. 2, P. 127, 1992).
In the optical magnetic field sensor composed as shown in FIG. 2, an optical fiber 9 employs multi-mode fibers having a core size 80 .mu.m, and a lens 12 employs self-focussing rod lens having a pitch 0.25. The polarizer 13 and the analyzer 15 employ polarization beam splitters; and a full-reflecting mirror 16 is used to bend an optical path by 90 degrees. The polarization beam splitters and the full-reflecting mirror are cubes whose side each is 5 mm. The magneto-optical element 14 employs a rare-earth iron garnet crystal. A system using the magnetic field measuring equipment to which such principle is applied has been proposed in which magnetic field measuring units are arranged at a plurality of points in a transmission/distribution line; the electrical outputs from each measuring unit are inputted to an arithmetic unit, where the sum of or difference among the outputs with respect to their waveform is taken as a reference signal; and for example, a zero-phase current in the transmission/distribution line is detected to determine an accident.
However, where a magneto-optical element used for such optical magnetic field sensor employs a ferrimagnetic rare-earth iron garnet crystal, the magnetic domain specific to the rare-earth iron garnet crystal causes the light having transmitted the crystal to be diffracted. The diffracted light is defined as a zero-order light 24, a first-order light 25, a second-order light 26 and the like on a screen 28 from the center thereof, as shown in FIG. 14. In the optical magnetic field sensor composed as shown in FIG. 2, the detection condition of the diffracted light in the lens 12 on the light emitted side is substantially zero-order light detection, so that the output therefrom is expressed by the following equation (1) (see J. Mag. Soc. Jpn., Vol. 14, No. 4 P. 642, 1990): ##EQU1## wherein, .theta..sub.F is the Faraday rotation (saturation Faraday rotation) when a material is magnetically saturated and expressed by .theta..sub.F =F.multidot.L in which F is a coefficient of the Faraday rotation specific to the material and L is an optical path length (element length); M is a magnetization of the material when a magnetic field is applied to it; and Ms is a magnetization (saturation magnetization) when the material is magnetically saturated.
As a magneto-optical element used for the optical magnetic field sensor as described above, there has been disclosed a rare-earth iron garnet crystal which is expressed by a general formula (chemical formula 1), in which the value of X is set at X=1.3; that of Y at Y=0.1; that of Z at Z=0.1; and that of W at W=0.6 (see U.S. Pat. No. 5,212,446 or the technical report OQE.sub.92 -105, 1992, of The Institute of Electronics, Information and Communication Engineers). In the prior art, the substitution of Bi or Gd for Y allows a magneto-optical element having good temperature properties. A chemical formula of the crystal used for prior art examples is shown in (formula 3). EQU Bi.sub.x Gd.sub.y R.sub.z Y.sub.3-(x-y-z) Fe.sub.5-w Ga.sub.w O.sub.12(formula 3)
However, where an optical magnetic field sensor is composed, as shown in FIG. 2, by the use of the magneto-optical element, there has been indicated a linearity error of magnetic field measurement of .+-.2.0% or less within a range 5.0 Oe to 200 Oe, as shown in FIG. 4, so that there is a problem in practice with respect to the accuracy of a magnetic field measuring equipment.
The linearity error is due to a fact that where the magneto-optical element used for an optical magnetic field sensor employs a Bi-substituted rare-earth iron garnet crystal as described above, when a light is transmitted through the garnet crystal being a ferrimagnetic substance, a light diffraction occurs due to the magnetic domain structure of the garnet crystal, whereby the diffracted light having transmitted the crystal is not completely detected in an optical system on the light emitted side in which only the zero-order light is thus received.